In agricultural spraying, the flow rate through a spray nozzle is important in order to deliver the specified amount of active ingredient to a specified area. The proper flow rate is often a function of nozzle spacing and vehicle ground speed.
Liquid pressure across the spray nozzle can also be an important consideration. The pressure across a spray nozzle often regulates the distribution of sizes of the droplets being delivered. The distribution of droplet size and the application conditions can influence the target coverage and the occurrence of spray drift, where droplets are displaced by ambient wind and subsequently land outside of the designated spray area. Chemical type, plant canopy, and weather conditions often mandate the droplet size that is required for a particular spraying application.
Because the timing and rate of fluid flow and the desired pressure are derived from different parameters, the ability to control flow rate and pressure independently would be very advantageous. Specifically, the ability to quickly and precisely control the application rate and droplet size, at a high degree of spatial resolution, is important in achieving optimal pest control and environmental protection. Moreover, agricultural spraying is typically a low-margin business, as the spraying components used are often very expensive. Thus, the control of both instantaneous pressure and average flow rate using minimal components, such as by using a single actuator, would be desirable.
Typical liquid agrochemical application systems pressurize liquid from a reservoir and atomize the pressurized liquid streams into droplets through the spray nozzles. Spray nozzles may be selected to provide a range of droplet sizes, distribution patterns, and flow rates for a desired liquid application. Additionally, the pressure of the liquid supplied to the spray nozzles is typically regulated system-wide with an in-line or bypass valve, or through pump speed control. Moreover, in many conventional spraying applications, the pressure at an individual spray nozzle is considered only as it relates to the desired flow rate, wherein the flow rate is proportional to the square root of pressure. Consequently, large changes in pressure are required to make moderate flow rate changes. Also, the pattern or spatial distribution of the spray emitted from a spray nozzle is affected by the liquid pressure. A decrease in pressure will increase the droplet size and decrease the size of the spray pattern and the overlap of the spray patterns between nozzles. Often, at low pressures the pattern does not fully develop. This can result in incomplete coverage or excess coverage in portions of the same field.
Pulse width modulation (PWM) of the liquid supplied to each spray nozzle is an alternative to system pressure variation for flow control and is now a mature technology adopted in the U.S., Canada, and Australia. For example, known applications for PWM flow control systems are disclosed in U.S. Pat. No. 5,134,961 (Giles et al.), U.S. Pat. No. 5,653,389 (Henderson et al.), U.S. Pat. No. 7,311,004 (Giles) and U.S. Pat. No. 7,502,665 (Giles et al.) and U.S. Pat. Pub. Nos. 2006/0273189 (Grimm et al.) and 2010/0032492 (Grimm et al), all of which are hereby incorporated by reference herein in their entirety for all purposes.
In a PWM flow control system, the fluid pressure is essentially held constant at a desired value to achieve the desired droplet size spectrum and the fluid flow is interrupted in a continuously cyclic timed sequence by an actuator positioned at the nozzle inlet. Studies have shown that changes to droplet size distributions of modulated sprays are negligible and that PWM flow control methods may be used as a form of droplet size control. Because PWM flow control systems allow for flow rate changes at constant pressures, manipulation of the system pressure essentially acts as a system-wide droplet size controller.
Increasing efficiency in agricultural spray operations has trended toward machines with wider spray swaths and faster ground speeds. A side effect of these increased efficiencies is that very wide spray booms exaggerate application rate errors. One example of these rate errors exists if the spray boom is executing a turn. The spray nozzles on the inside of the turn travel slower than the spray nozzles at the distal end of the boom. Thus, some areas receive more material than desired resulting in higher chemical residues, crop damage, and wasted pesticide while other areas receive less material and a loss of pest control.
One of the advantages of modern PWM flow control systems is the capability of individual nozzle rate control due to the actuator positioned at each spray nozzle. The control of rate at per-nozzle resolution results in fewer application rate errors due to the ability to make adjustments to the rate at the highest possible spatial resolution. However, the quality of an application is dependent on numerous factors in addition to the quantity of material applied. Specifically, droplet size remains an important ingredient in maximizing efficacy and minimizing drift. Unfortunately, even with modem PWM flow control systems, droplet size is controlled in per-machine width resolution, that is, every nozzle across the entire boom must produce the same droplet size spectrum. There are many situations where adjusting the droplet size on an individual nozzle may be desirable, including use in narrow buffer zones where a larger droplet size is mandated for mitigating spray drift. With current technology, a spray machine with a 120-foot boom is required to maintain large droplets on a 120-foot swath, even if the mandated buffer zone is only 10 feet. This application would reduce the efficacy of the application in areas not at risk of drift. Modern global positioning systems with centimeter-scale accuracy and high-speed computers are capable of implementing very precise applications for diverse crops in close proximity. To gain full use of these sensor and processing technologies, the need for high treatable spatial resolution is increasing. However, one of the primary limitations to precision agricultural technology is the lack of high-quality actuation systems to match the precision of today's advanced sensing systems. Accordingly, there is a need for a system that can provide uniform spray distribution with individual control of flow rate and droplet size distribution of the spray emitted from each spray nozzle.
U.S. Pat. Pub. No. 2008/0230624 (Giles and Needham), which is hereby incorporated by reference herein in its entirety for all purposes, discloses a spray actuator that controls the flowrate of liquid through a valve and the pressure drop across the valve during the instantaneous flow. While this spray actuator provided a basic idea for instantaneous pressure and average flow control, significant improvements in actuator form and function are needed. To illustrate, the characteristics of the spray actuator made pressure control susceptible to fluctuating supply voltage, fluctuating valve inlet pressure, and aging of rubber seals. In addition, the spray actuator exhibited a lack of linearity of control. Improvements to the valve geometry, control algorithms, diagnostics, and applications of the single actuator concept would be welcomed in the technology.
In light of the above highlighted design concerns in the field of agricultural spraying, a need continues to exist for refinements and improvements to address such concerns. While various implementations of spray nozzles, nozzle assemblies, spraying systems and control systems have been developed, no design has emerged that is known to generally encompass all of the desired characteristics hereafter presented in accordance with aspects of the present subject matter.